Page:Popular Science Monthly Volume 73.djvu/491

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487
THE CAUSE OF PULSATION

Thus the sodium oxalate which forms in the sense-organs is simply changed into ordinary table salt, which acts as a stimulus to produce pulsation.

We can prove experimentally that this suffices to explain the phenomenon of pulsation, for if we simply add from 1 to 5 parts of common salt to 1,000 parts of sea-water, we find that this slight excess of salt acts as a powerful stimulant to the sense-organs, but produces no pulsation if placed upon other parts of the jellyfish.

It thus appears that each sense-organ normally maintains a certain excess of common salt which acts as a stimulus, and which is prevented from becoming too concentrated by the fact that being soluble it is constantly dissolving out into the surrounding sea-water.

It may trouble us for a moment to see why a recurrent pulsation should arise from a constantly present stimulus, but long ago Romanes discovered that a weak constantly present stimulus, such as a faradaic current of electricity, will cause rhythmical pulsation, the jellyfish responding to it periodically and regularly.

We see then that the natural stimulant which produces the pulsation of the jellyfish is only that most familiar substance common salt!

The hearts of higher animals behave in a manner so similar to that of the pulsating jellyfish that we need not be surprised if it be demonstrated that here also a slight excess of sodium chloride gives rise to each and every pulsation.